Porous materials are of paramount importance in many engineering disciplines. For centuries, woven fabrics have been applied to numerous filtration operations with the woven matrix impeding the passage of large suspended matter while permitting the flow of the suspending media. More recently, non-woven membranes characterized by tightly controlled pore size and distribution have been developed for use in ultrafiltration and reverse osmosis operations. These materials have a parallel pore network consistent with their flow-through objective.
Other porous structures have been designed for light weight structural support duty and are often applied in aerospace and aircraft construction, e.g., wing design. In these structures, the focus is on the managed translation of forces and loads throughout the structure. The ancillary network of pores associated with these designs are solely directed to the support of certain forces while minimizing the associated mass.
In the medical field, there has recently developed a need for porous articles for use in conjunction with certain medical procedures. These procedures include reconstructive surgery and repairing of highly comminuted fractures. The healing of a bone break can be accelerated by the inclusion of porous matrix adjacent to the break point, enhancing bone growth. In reconstructive surgery, a damaged bone, such as a tibia, is rebuilt. This often requires the insertion of a suitable prosthesis device to provide the desired shape.
A prosthesis device is an engineered element that achieves certain biological functions when placed within a living organism. An important class of prosthesis devices is directed to repair and/or replace human body elements, such as knee or hip joints. To replace a biological joint in an acceptable long lasting manner, the replacement element must join with the surrounding tissue. This is also true in applications of artificial skin and artificial blood vessels. The proper melding of the prosthesis is accomplished through the use of an appropriate material having a micro-network of capillaries permeating the structure to permit tissue in-growth. The porous network must be continuous, permitting unrestricted passage of blood and linkage to the surrounding tissue, while providing a degree of structural support. This can be easily envisioned in the design of artificial blood vessels, wherein the vessel wall must support the forces associated with blood flow while simultaneously passing oxygen, proteins and other extracellular matter to the surrounding tissue.
The porous materials providing the best blend of performance characteristics were first found in nature. Certain aquatic animals were discovered as having a micro-porous matrix. More specifically, the protoreaster (spiny starfish), slate pencil sea urchin and certain coral exhibit a solid structure formed of calcium carbonate having a network of interconnecting pores and significant void volume. For example, the slate pencil sea urchin has cigar-shaped protrusions that have a void volume of 50 percent, a porous structure with pore diameters of approximately 25 .mu.m., and a mostly uniform structure exhibiting a mathematically minimal surface area. Certain coral provide similar attributes with pore diameters of approximately 250-600 .mu.m.
In the past, these aquatic materials were used to form biologically acceptable structures by injecting silicone rubber into the porous matrix and then dissolving the calcium carbonate skeleton. Another technique involved the hydrothermal treatment of the calcium carbonate skeleton, forming hydroxyapatite (HA). A more detailed discussion of these techniques may be found in U.S. Pat. Nos. 3,929,921, 3,890,107, 4,231,979, 4,861,733 and 4,722,870; the teachings of which are incorporated herein by reference. Although these procedures offer a unique class of structures, they are accompanied by several significant drawbacks. As a first point, the naturally forming aquatic structures were never completely uniform and often exhibited imperfections detrimental to surgical implantation. In addition, the materials are expensive to harvest and have raised certain environmental concerns.
These problems have sparked a search for techniques to engineer and manufacture porous materials having specifically delineated structural properties in a controlled manner. The present invention is a result of this search.